Since I somewhat evaded the two questions...

How much is a simple answer - as much as needed to illustrate success. Resistance can be a positive feedback and a negative one. For example, I can use resistance to see if I am balanced. Or, I can use resistance to test the quality of my movement.

When is during resistance training. Not during learning. If you don't know what you're doing, not knowing what you're doing with resistance does not change that equation.